Ion microprobe analyses of δ18O in early quartz cements from 1.9 Ga granular iron formations (GIFs): A pilot study

The low δ18O values of Precambrian cherts have been widely used to infer that temperatures were higher and/or seawater δ18O was lower compared to today's oceans. However, the Precambrian cherts presented as evidence for these temperatures are neomorphosed from amorphous precursors that originally precipitated from ocean water, suggesting diagenetic alteration is an important cause of the widespread low-δ18O values. Many pores between sand-size clasts in granular iron formations (GIFs) are filled with coarsely crystalline quartz cements that are texturally primary. In unmetamorphosed GIF samples from the 1.9 Ga Gunflint and Sokoman iron formations, synsedimentary clasts of GIF and high minus-cement porosities (the volume of pore space occupied by recognizable cement, inversely proportional to the degree of compaction) indicate such cements precipitated directly from pore water near the depositional interface. Growth bands identified in quartz cement crystals using cathodoluminescence (CL) imaging further confirm their void-filling and unrecrystallized nature. In order to test the effect of neomorphism on the δ18O of silica in these rocks, unrecrystallized primary quartz cements and neomorphic quartz in adjacent clasts were both analyzed by ion microprobe. Values of δ18O range from 23.5 to 26.4‰ VSMOW in cements and 21.3-26.8‰ in clasts in samples from the Gunflint Iron Formation. Samples from the Sokomon Iron Formation record deeper, more evolved pore waters and in some cases contain lower δ18O cements. In individual pores of Gunflint samples, cement values vary only slightly more than analytical uncertainty from edge to center and no consistent increasing or decreasing trends were observed. Overall, the δ18O of the cements are similar to the most elevated δ18O reported from cherts of similar age, but span a substantially smaller range. This suggests most of the lower values reported previously reflect late-diagenetic conditions at greater burial depths. The shallow, possibly restricted seas above the Gunflint and Sokoman GIFs appear to have been warmer and may have been lower in δ18O than present-day seawater, but they may not have been representative of global seawater.